As a method replacing conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has been practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the light emission to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The radiation image storage panel thus treated is subjected to a step for erasing a radiation image remaining therein, and then is stored for the next radiation image recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen. The radiation image recording and reproducing method using a stimulable phosphor is of great value especially when the method is employed for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. The stimulable phosphor layer generally comprises stimulable phosphor particles and a binder polymer. Further, a transparent film of polymer material is generally provided on the free surface (i.e., surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
As well as a phosphor layer comprising a binder and a stimulable phosphor dispersed therein, a phosphor layer formed by deposition process or firing process can be employed.
The radiation image recording and reproducing method can be performed by means of an apparatus comprising: recording means (by which a radiation image is recorded on the panel), reading means (by which the radiation image recorded in the panel is read through the steps of exciting the stimulable phosphor with a stimulating ray to release stimulated emission and photoelectrically detecting the emission to read the recorded image), erasing means (by which the radiation image remaining on the panel is erased with erasing light), and conveying system connecting each means for conveying the panel. In such all-in-one type apparatus, the panel is repeatedly conveyed and repeatedly used. The above means may be separated into a recording apparatus comprising the recording means and a reading apparatus which has the reading means and the erasing means. In such case, the method is performed by a combination of the recording apparatus and the reading apparatus. The radiation image storage panel is repeatedly used in either case.
In the radiation image recording and reproducing method, the radiation image recorded in the storage panel is generally read by applying the stimulating rays onto one surface side of the storage panel and collecting light emitted by the phosphor particles by means of a light-collecting means from the same side. However the light emitted by the phosphor particles may be collected on both sides of the radiation image storage panel. For instance, it may be the case that the emitted light is desired to be collected as much as possible. There also is a case that the radiation image recorded in the phosphor layer varies along the depth of the layer and such variation is desired to be detected. A typical radiation image reading system reading from both sides (hereinafter, referred to as "double-side reading system") is illustrated in the attached FIG. 1.
In the FIG.1, the radiation image storage panel 11 is transferred (or moved) by a combination of two sets of nip rolls 12a, 12b. The stimulating rays such as laser beam 13 is applied onto the storage panel 11 on one side, and the light emitted by the phosphor particles in the storage panel advances upward and downward (in other words, to both the upper and lower surface sides). The downward advancing light 14a is collected by a light collector 15a (arranged on the lower side), converted into an electric signal in a photoelectric conversion device (e.g., photomultiplier) 16a, multiplied in multiplier 17a, and then sent to a signal processor 18. On the other hand, the upwardly advancing light 14b is directly, or after reflection on a mirror 19, collected by a light collector 15b (arranged on the upper side), converted into an electric signal in a photoelectric conversion device (e.g., photomultiplier) 16b, multiplied in multiplier 17b, and then sent to a signal processor 18. In the signal processor 18, the electric signals sent from the photoelectric conversion devices 17a, 17b are processed in a predetermined manner such as addition or reduction of the signals depending on the nature of the desired radiation image.
The radiation image storage panel 11 is further moved by means of two sets of nip rolls 12a, 12b in the direction indicated by the arrow. The surface area of the panel on which the stimulating rays 13 have been applied is then set under a light source 20 such as a sodium lamp 20 for erasing an radiation image remaining in the storage panel 11.
As is described above, the radiation image storage panel is repeatedly used in the cyclic procedure comprising the steps of exposing to a radiation (for recording of a radiation image), irradiating with stimulating rays (for reading of the recorded image) and exposing to an erasing light (for erasing the remaining image). The storage panel is transferred from one step to another step by means of conveying means such as belt and rolls, and after a cycle of steps is conducted, the panel is piled up on other panels and stored for next cycle.
The radiation image storage panel used in the double-side reading system generally has a phosphor layer whose faces are covered with a transparent support (on the bottom side) and a transparent protective film (provided on the top side). However, the present inventors have found that such panel has a disadvantageous property: that is, the panel having been repeatedly used many times often gives relatively poor radiation images. For example, a ghost image is superimposed on the desired image, or noises have occurred which make the image quality poor.
The inventors have studied the cause of production of poor radiation image and found the mechanism of the deterioration of the radiation image: that is, as the panel is repeatedly used, stains gradually deposit and abrasions are produced on the protective film and the back surface (surface not facing the phosphor layer) of the support, and therefore such stains and abrasions disturbs passages of the emitted light and make the image quality lower.
With respect to the radiation image storage panel used in the single-side reading system, such deterioration is known and several improvements are proposed. In U.S. patent application Ser. No. 08/469,761 for example, the use of a protective film of a resin containing a fluororesin soluble in an organic solvent is proposed. Also proposed is a protective film made of a resin containing a film-forming resin and oligomer having a polysiloxane structure and/or having a perfluoroalkyl group in U.S. Pat. No. 5,227,253. U.S. patent application Ser. No. 08/834,772, now issued as U.S. Pat. No. 5,866,266, describes that a protective composite of a plastic film and a coated film of a fluororesin composition are placed on the phosphor layer.
There is no knowledge, however, about the mechanism of deterioration of the radiation image given by the storage panel having been repeatedly used in the double-side reading system.